Separating impurities from a liquid stream is a difficult and necessary task in many industries such as, for instance, in the oil industry where a produced liquid stream must be treated to remove dissolved impurities and metals. Conventional techniques for treating such a liquid stream have included the use of chemical additives to bring the impurities together and filtration and/or dissolved air floatation to separate and remove the impurities from the liquid. However, these prior techniques have their own set of inherent problems. For instance, the added chemicals can result in large quantities of chemical sludge, which itself must be treated before being disposed of in the environment. Filtering techniques can result in filter clogging, low separation rates, and low overall efficiencies.
Dissolved air floatation is a technique wherein bubbles are introduced into and mixed thoroughly in the wastewater and these bubbles attach themselves to particles of impurities. As the resulting mixture rests in, for instance, a holding tank, the bubbles float to the surface carrying the impurities with them. The cleaned wastewater can then be extracted from the bottom of the holding tank while the impurities are skimmed from the surface of the water in the tank. Dissolved air floatation, while promising, comes with its own set of inherent problems. For example, it is difficult to produce bubbles that are sufficiently small to attach themselves to very small contaminant particles, and so the process can be inefficient for such particles. Perhaps more salient, however, is the fact that the pressure that necessarily is used to pump the air into the fluid results in bubbles with internal pressures higher than that of the surrounding liquid. This, in turn, produces bubbles with relatively high surface tension, which resists attachment of the bubbles to contaminant particles. For at least these reasons, dissolved air floatation has heretofore enjoyed limited success.
Accordingly, there is a long felt and continuing need for a technique and apparatus for separating impurities from a stream of wastewater using dissolved air floatation that is more efficient and more effective than prior art techniques. The technique and apparatus should produce in the wastewater bubbles of arbitrarily small size to attach to very small particles of impurities and with the lowest possible surface tension to enhance the attachment process. The present invention seeks to fill this need.